The Mask Mesh is a separate mesh file from the Subject Mesh. It should contain any geometry in within the scene that you do not wish to be part of the final asset. This includes any supports or artifacts in the photogrammetry solveoccludes the subject from a given camera view.
In this example, the plinth occludes the shoe for the lower cameras.
To avoid this, use a separate ‘Mask Mesh' to denote unwanted geometry that occludes the subject, in this case, the plinth. Blue will be used for the final texture, Red will not.
Make sure the Subject is not present in the mask mesh, otherwise no textures will be produced.
Providing a mask mesh ensures that these areas will not affect the final asset, as otherwise they can be projected across the surface, leading to problems in the final textures.